A COG-WHEEL MODEL FOR NUCLEAR-SPIN PROPAGATION IN SOLIDS

An efficient quantum-mechanical approach is presented for the computat ion of the short-term propagation of nuclear polarization in many-spin systems, known as spin diffusion, which carries structural informatio n. It is based on the fact that for short time intervals the spin evol ution under dipolar coupling can be expressed analytically by rotation s in three- and four-dimensional subspaces of the Liouville space. Due to its high computational efficiency, the approach allows the treatme nt of several dozens of coupled spins and is well suited to the rapid calculation of spin propagation in oriented samples and in powders. (C ) 1997 Academic Press.